Arsenal lost 2-1 to Brentford in a behind-closed-doors friendly at the Gunners' London Colney training complex on Wednesday.
Mikel Arteta's men had won all five of their pre-season friendlies so far this summer, but fell to Thomas Frank's side ahead of the returning Emirates Cup this weekend.
Both teams used largely second string lineups, but it was a Brentford regular who opened the scoring nine minutes in. Fin Stevens managed to release Ryan Trevitt into space, and his cross was turned in by Bryan Mbeumo.
Arsenal similarly scored through one of their more senior players when Eddie Nketiah equalised midway through the first half, tapping in after some neat interplay from Reiss Nelson and Ainsley Maitland-Niles.
Summer signing Marquinhos twice went close for the Gunners as he attempted to impress Arteta, but Brentford notched themselves back in front on the stroke of half-time when Rico Henry's low cross was slotted home by Trevitt.
Nketiah and Albert Sambi Lokonga both went close for Arsenal on the other side of the break, while Maitland-Niles and Nuno Tavares were both denied by substitute goalkeeper Matthew Cox.
Cedric went closest to finding an equaliser for the north London side late on but Brentford weathered the storm to pick up their second pre-season win in two days having already beaten Brighton on Tuesday.
Arsenal face Sevilla in the Emirates Cup on Saturday afternoon before opening the Premier League season away at Crystal Palace next Friday. Brentford host the Andalusian's city rivals Real Betis later on Saturday and begin their campaign away at Leicester next Sunday.
Arsenal (4-3-3): Turner, Cedric, Walters (Awe 46), Holding, Tavares (Souza 76), Elneny, Lokonga, Maitland-Niles (Patino 85), Marquinhos, Nelson (Sagoe Jr 90), Nketiah
Brentford (4-3-3): Strakosha (Cox 62), Stevens, Bech, Crama, Henry (Racic 62), Onyeka, Ghoddos (Yarmolyuk 75), Peart-Harris, Trevitt, Lewis-Potter, Mbeumo (Gilbert 75)